From a numerical viewpoint, the religion of most Americans has its roots in an ancient population explosion.
In the span of 300 years, a tiny group of early Christians expanded to absorb an empire. Students of antiquity have been asking ever since how it happened.
With a new interpretive tool, sociologist Rodney Stark has argued that it was a more common-sense dynamic than first meets the eye. As in any religion that proliferates, he said, people join by families and friendships. Then, they stay by a "rational choice" of costs and benefits.
The arithmetic approach has offended some church historians and believers. But Mr. Stark also is making points by applying "rational choice" theory to religion, as in his newest book, "The Rise of Christianity."
"If you take this principle and go back to the early church, social science sees it with very different eyes," Mr. Stark said in an interview. "When numbers run into stories, numbers win."
As viewed by history's stories, Christianity rose on martyrdoms, catacombs, slave revolts or, to the cynical historian, social hysteria as the empire crumbled.
To view it mathematically, Mr. Stark said, may be to emphasize birth rates over faith, but also to re-establish primacy of belief.
First, early Christianity could not have succeeded if it were only an underground network. "If you can't find this new religious movement, how is it going to grow?" he said. "If it's totally secret, it's not going to happen."
Looking at the ancient sources, he sees an open promulgation of the new faith. Next comes the head count.
While the Book of Acts states that at mid-century "many thousands" had become believers, early church fathers contradict that with lower numbers.
Given the imprecision, Mr. Stark uses the most credible data. He sets his brackets at 1,000 Christians in A.D. 40, growing to between 5 million and 7.5 million believers in A.D. 300.
In between, Christian membership had to have enlarged by 40 percent every decade - that is, for every five Christians, two more join by conversion, marriage or birth. …